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Spring wheat (Triticum aestivum L.) is a major crop of the northern Great Plains. High temperature stress during the grain‐fill period is a major constraint to increased productivity of spring wheat, particularly in the southern part of the region. The electrolyte leakage or membrane thermostability (MT) test was conducted at anthesis on flag leaves of eight field‐grown genotypes of spring wheat and the results related to the yield of those genotypes grown at three other field locations. Genotypes were derived from two crosses involving parents adapted and unadapted to the region or thermal aspects of the climate. The three field locations represented northern, central, and southern areas of the northern Great Plains with soil classifications of Aeric Calciaquoll, Aeric Calciaquoll, and Typic Haplustoll, respectively. Based on MT values, genotypes were grouped as heat‐tolerant (HT, n = 4) vs. heat‐sensitive (HS, n = 4). The HT and HS genotypes produced similar grain yields at the northern and central locations. However, the HT group of genotypes produced 21% more grain yield (4.28 Mg ha−1) than the HS group (3.55 Mg ha−1) at the southern site, which was considerably warmer during the grain‐fill period than the other two locations. Average temperatures during July were 19.5, 21.7, and 23.4 °C at the northern, central, and southern locations, respectively. Based on results of this study it seems that the MT test is a useful screening procedure for selecting HT genotypes of spring wheat.

Spring wheat (Triticum aestivum L.) can be either daylength sensitive or insensitive. Sensitive wheat require long days for timely flowering, whereas insensitive wheat mature normally even in short days. The objective of this study was to compare near‐isogenic daylength insensitive and sensitive hard red spring wheat lines to determine whether insensitivity had adverse effects on agronomic traits and grain protein. Eleven pairs of phenotypically diverse hard red spring wheat lines near isogenic for daylength response were evaluated in 25 environments in the upper Midwest. The grain yield for insensitive lines of all pairs was the same as or higher than for the sensitive lines, averaged over all environments. Yield differences between paired insensitive and sensitive lines varied for specific environments, but a general yield disadvantage for insensitive lines was not observed in any environment. Insensitive lines tended to outyield their sensitive counterparts in environments with lower mean yields or shorter grain fill periods. The insensitive lines were earlier in heading than the sensitive lines for most but not all pairs. Grain protein content for insensitive lines was the same as or lower than for sensitive lines, but selection of insensitive lines with adequate protein should not be difficult. The incorporation of insensitivity appears to result in genotypes generally well adapted to the upper Midwest, and may have beneficial effects on some agronomic traits.

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